MOSCOW -- An improvised memorial near the Kremlin where Russian opposition politician Boris Nemtsov was gunned down nearly a year ago has been vandalized for a fourth time since the beginning of the year.

Activists Sergei Kolesnikov and Karine Kovkhayeva said on February 17 that they were at the site overnight when police took them away for questioning.

After they returned 90 minutes later, all the flowers had been removed from the spot, they added.

The activists preserved Nemtsov's portraits, which they reinstalled at the site.

Nemtsov was fatally shot on a bridge near the Kremlin on February 27, 2015.

Several men from the North Caucasus region have been arrested and charged with the killing.

Nemtsov's relatives and lawyers have expressed skepticism about the probe, insisting the killing must have been ordered by high-ranking Russian officials.